The word "angel" can be traced back to the ancient Greek "angelos", or messenger, which referred to a specific class of angel, but in modern times the term has been extended to all spirits belonging to the heavenly host. Besides the popularized bird-winged-humanoid form of angels, angels can appear as wheels covered with eyes, as multi-headed beasts, or even geometric shapes.
Most angels show unwavering fealty to God, but there are many who chose other paths such as Lucifer and Satan. Other angels can have brutally wicked dispositions despite their heavenly favor, including Metatron, Mastema, Abaddon, and Samael.
The perception of angels as seen in Judaism and into the early Christian church was likely developed from a mix of Babylonian, Zoroastrian, and perhaps even Greek influences.